In drilling wells for oil and gas exploration, understanding the structure and properties of the geological formation provides information to aid such exploration. Due to the difficult and expensive process of drilling boreholes that is used to gain direct access to the formations, remote reservoir scale sensing methods are utilized to make images of the formations from the surface. The three most popular imaging methods are seismic, which is based on acoustic waves; controlled source electromagnetic (CSEM), which is based on electromagnetic waves; and crosswell electromagnetic tomography (crosswell).
Although the seismic method is widely used, it is only sensitive to mechanical properties of the formations and cannot easily differentiate waterbearing formations from oil-bearing formations. CSEM and crosswell methods, on the other hand, provide electromagnetic measurements that are sensitive to presence of water, since water creates a significant contrast in formation resistivity. However, both CSEM and crosswell methods suffer from low resolution due to conductive losses and the dispersive characteristics of formations at low frequencies. To overcome this difficulty, a priori information, perhaps obtained from a pilot well, is often used to produce an initial guess of formation properties between the wells used to gather measurements. However, the use of a priori information often produces a biased, and consequently unreliable, result.